If you want to have an additional battery in any case, then head for the mr3020!
The MR3040 tend to not load quick enough to reload the internal battery while being loaded. So even with a full external battery, you mr3040 run into troubles because of having a low internal battery as power source.
The MR3040 tend to not load quick enough to reload the internal battery while being loaded. So even with a full external battery, you mr3040 run into troubles because of having a low internal battery as power source.